Precision Calculations for the LHC

Transcription

1 Precision Calculations for the LHC LHC Olympics 2006 Zvi Bern, UCLA with Carola Berger, Lance Dixon, Darren Forde and David Kosower hep-ph/ hep-ph/ hep-ph/ hep-ph/ hep-ph/

2 Outline The need for NLO calculations at the LHC The experimenters wish list. The trouble. On-shell methods. Rewrite of QFT. Example of state-of-the art calculations Blackboard lecture, working through a simple example of on-shell recursion. 2

3 LHC Physics The LHC will start operations in We will have lots of multi-particle processes. Want reliable predictions. 3

4 Example: Susy Search Early ATLAS TDR studies using PYTHIA overly optimistic. ALPGEN vs PYTHIA ALPGEN is based on LO matrix elements and much better at modeling hard jets. What will disagreement between ALPGEN and data mean? Hard to tell. Need NLO. 4

5 Example: Higgs + 2 jets from Weak Boson Fusion Figy, Oleari and Zeppenfeld Berger and Campbell Campbell, Ellis and Zanderighi Purpose: After discovery of Higgs Boson measure HWW coupling Background uncertainty is reduced with an NLO calculation. 5

6 LHC Experimenter s NLO Wishlist Les Houches 2005 Large number of high multiplicity processes that we need to compute. Numerical approaches. Recent progress. Binoth and Heinrich Kaur; Giele, Glover, Zanderighi; Binoth, Guillet, Heinrich, Pilon, Schubert; Soper and Nagy; Ellis, Giele and Zanderighi; Anastasiou and Daleo; Czakon; Binoth, Heinrich and Ciccolini In this talk analytic on-shell methods: spinors, twistors, unitarity method, on-shell bootstrap approach. Bern, Dixon, Dunbar, Kosower; Bern and Morgan; Cachazo, Svrcek and Witten; Bern, Dixon, Kosower; Bedford, Brandhuber, Spence, Travaglini; Britto, Cachazo, Feng and Witten; Berger, Bern, Dixon, Kosower, Forde; Xiao, Yang and Zhu 6

7 Example of difficulty Consider a tensor integral: Evaluate this integral via Passarino-Veltman reduction. Result is 7

8 Result of performing the integration Numerical stability is a key issue. Clearly, there should be a better way 8

9 What we need Numerical stability. Scalable to large numbers of external partons. A general solution that applies to any process. Can be automated. What we re dreaming of A technique where computations undergo modest growth in complexity with increasing number of legs. Compact analytic expressions that fit on a page. 9

10 Progress Towards the Dream Results with on-shell methods: Many QCD amplitudes with legs. Certain log contributions via on-shell recursion. Improved ways to obtain logarithmic contributions via unitarity method. Berger, Bern, Dixon, Forde and Kosower Bern; Bjerrum-Bohr; Dunbar, Ita Britto,Cachzo, Feng; Britto, Feng and Mastrolia Key Feature: : Modest growth in complexity as n 10 increases. No unwanted Gram dets.

11 Spinors and Twistors Spinor helicity for gluon polarizations in QCD: Witten Penrose Twistor Transform: Witten s remarkable twistor-space link: Early work from Nair Witten; Roiban, Spradlin and Volovich QCD scattering amplitudes Topological String Theory Key implication: Scattering amplitudes have a much much simpler structure than anyone would have believed. 11

12 Amazing Simplicity Witten conjectured that in twistor space gauge theory amplitudes have delta-function support on curves of degree: Connected picture Structures imply an amazing simplicity in the scattering amplitudes. MHV vertices for building amplitudes Cachazo, Svrcek and Witten Disconnected picture Witten Roiban, Spradlin and Volovich Cachazo, Svrcek and Witten Gukov, Motl and Neitzke Bena, Bern and Kosower Much of the simplicity survives addition of masses or loops 12

13 Why are Feynman diagrams clumsy for high-multiplicity processes? Vertices and propagators involve gauge-dependent off-shell states. Origin of the complexity. To get at root cause of the trouble we must rewrite perturbative quantum field theory. All steps should be in terms of gauge invariant on-shell states. Radical rewriting of perturbative QCD needed. 13

14 One of the most remarkable discoveries in elementary particle physics has been that of the existence of the complex plane. J. Schwinger in Particles, Sources and Fields Vol 1 14

15 On-shell Formalisms With on-shell formalisms we can exploit analytic properties. Curiously, a practical on-shell formalism was constructed at loop level prior to tree level: unitarity method. (1994) Bern, Dixon, Dunbar, Kosower Solution at tree-level had to await Witten s twistor inspiration. (2004) -- MHV vertices -- On-shell recursion Britto, Cachazo, Feng, Witten Cachazo, Svrcek Witten; Brandhuber, Spence, Travaglini Combining unitarity method with on-shell recursion gives loop-level on-shell bootstrap. (2006) Berger, Bern, Dixon, Forde, Kosower 15

16 Unitarity Method Bern, Dixon, Dunbar and Kosower Two-particle cut: Three- particle cut: Generalized unitarity: Bern, Dixon and Kosower As observed by Britto, Cachazo and Feng quadruple cut freezes integral: Coefficients of box integrals always easy. Generalized cut interpreted as cut propagators not canceling. A number of recent improvements to method Britto, Buchbinder, Cachazo and Feng; Berger, Bern, Dixon, Forde and Kosower; Britto, Feng and Mastrolia; Xiao, Yang and Zhu 16

17 Early On-Shell Bootstrap Bern, Dixon, Kosower hep-ph/ Early Approach: Use Unitarity Method with D = 4 helicity states. Efficient means for obtaining logs and polylogs. Use factorization properties to find rational function contributions. Key problems preventing widespread applications: Difficult to find rational functions with desired factorization properties. Systematization unclear key problem. 17

18 Tree-Level On-Shell Recursion New representations of tree amplitudes from IR consistency of oneloop amplitudes in N = 4 super-yang-mills theory. Using intuition from twistors and generalized unitarity: Bern, Del Duca, Dixon, Kosower; Roiban, Spradlin, Volovich Britto, Cachazo, Feng BCF + Witten A n A n-k+1 Ak+1 On-shell conditions maintained by shift. Proof relies on so little. Power comes from generality Cauchy s theorem 18 Basic field theory factorization properties

19 Construction of Loop Amplitudes Berger, Bern, Dixon, Forde, Kosower Shifted amplitude function of a complex parameter Shift maintains on-shellness and momentum conservation Use unitarity method (in special cases on-shell recursion) Use on-shell recursion Use auxiliary on-shell recursion in another variable hep-ph/

20 Loop-Level Level Recursion z New Features: Presence of branch cuts. Unreal poles new poles appear with complex momenta. Pure phase for real momenta Double poles. Spurious singularities that cancel only against polylogs. Double count between cut and recursive contributions. On shell bootstrap deals with these features. 20

21 One-Loop Five-Point Example The most challenging part was rational function terms at the end of chain of integral reductions. Assume we already have log terms computed from D = 4 cuts. Only one non-vanishing recursive diagram: Only two double-count diagrams: Tree-like calculations These are computed by taking residues Rational function terms obtained from tree-like calculation! No integral reductions. No unwanted Grim dets. 21

22 Six-Point Example Rational function parts of scalar loops were by far most difficult to calculate. Using on-shell bootstrap rational parts are given by tree-like calculations. No integral reductions. 22

23 Numerical results for n gluons Choose specific points in phase-space see hep-ph/ Scalar loop contributions 6 points amusing count 7 points 8 points + 3,017,489 other diagrams Modest growth in complexity as number of legs increases At 6 points these agree with numerical results of Ellis, Giele and Zanderighi 23

24 What should be done? Attack items on experimenters wishlist. Automation for general processes. Assembly of full cross-sections, e.g., Catani-Seymour formalism. Massive loops -- tree recursion understood. First principles derivation of formalism: Large z behavior of loop amplitudes. General understanding of unreal poles. Connection to Lagrangian Badger, Glover, Khoze, Svrcek Vaman and Yao; Draggiotis, Kleiss, Lazopolous, Papadopoulos 24

25 Other Applications On-shell methods applied in a variety of problems: Computations of two-loop 2 to 2 QCD amplitudes. Bern, Dixon, Kosower Bern, De Frietas, Dixon Ansatz for planar MHV amplitudes to all loop orders in N = 4 super-yang-mills. Anastasiou, Bern, Dixon, Kosower Bern, Dixon, Smirnov Cachazo, Spradlin and Volovich Applications to gravity. Bern, Dixon, Dunbar, Perelstein and Rozowsky Bern, Bjerrum-Bohr and Dunbar Brandhuber, Spence and Travaglini Cachazo and Svrcek 25

26 Summary We need bold action to provide the full range of NLO calculations for the LHC. On-shell bootstrap unitarity and factorization. New results for one-loop n 6 gluons. Explicit numerical results for up to eight gluons. Technology should apply as is to external mass cases. Important issues remain: automation, massive loops, construction of physical cross sections with experimental cuts. Experimenters wish list awaits us! 26

27 Experimenter s Wish List Les Houches

28 Complete List 28

29 Extra Transparancies 29

30 Incorrect Plot from ATLAS TDR Background is actually 10 times larger. 30